1. Field of the Invention
The present invention relates to a columnar Ni-base heat-resistant alloy which exhibits high resistance to interganular corrosion at high temperature, capable of providing cast articles having sound surfaces and internal structure. More particularly, the present invention is concerned with a large-size cast article, in particular a large-size turbine blade, having sound surfaces and internal structure and exhibiting superior intergranular corrosion at high temperature, made by casting from the Ni-base heat-resistant alloy.
2. Description of the Background
It is well known that blades of dynamic machines, such as rotor and stator blades of gas turbines, rotor blades of hot-gas blowers and so forth, are made by casting from Ni-base heat-resistant alloys. For instance, Japanese Patent Laid-Open No. 6-57359 discloses the following Ni-base heat-resistant alloys (a) to (d), as materials suitable for rotor and stator blades of gas turbines and rotor blades of hot-gas blowers:
(a) An Ni-base heat-resistant alloy possessing superior strength, oxidation resistance and corrosion resistance at high temperature, having a composition containing, by weight: Cr: from 13.1 to 15.0%, Co: from 8.5 to 10.5%, Mo: from 1.0 to 3.5%, W: from 3.5 to 4.5%, Ta: from 3.0 to 5.5%, Al: from 3.5 to 4.5%, Ti: from 2.2 to 3.2%, C: from 0.06 to 0.12%, B: from 0.005 to 0.025%, Zr: from 0.010 to 0.050%, Mg and/or Ca from 1 to 100 ppm, and the balance substantially Ni and incidental impurities; PA1 (b) an Ni-base heat-resistant alloy possessing superior strength, oxidation resistance and corrosion resistance at high temperature, having a composition containing, by weight: Cr: from 13.1 to 15.0%, Co: from 8.5 to 10.5%, Mo: from 1.0 to 3.5%, W: from 3.5 to 4.5%, Ta: from 3.0 to 5.5%, Al: from 3.5 to 4.5%, Ti: from 2.2 to 3.2%, C: from 0.06 to 0.12%, B: from 0.005 to 0.025%, Zr: from 0.010 to 0.050%, Hf: from 0.2 to 1.5%, Mg and/or Ca from 1 to 100 ppm, and the balance substantially Ni and incidental impurities; PA1 (c) an Ni-base heat-resistant alloy possessing superior strength, oxidation resistance and corrosion resistance at high temperature, having a composition containing, by weight: Cr: from 13.1 to 15.0%, Co: from 8.5 to 10.5%, Mo: from 1.0 to 3.5%, W: from 3.5 to 4.5%, Ta: from 3.0 to 5.5%, Al: from 3.5 to 4.5%, Ti: from 2.2 to 3.2%, C: from 0.06 to 0.12%, B: from 0.005 to 0.025%, Zr: from 0.010 to 0.050%, Hf: from less than 1.5%, Mg and/or Ca from 1 to 100 ppm, one, two or more of Pt: from 0.02 to 0.5%, Rh: from 0.02 to 0.5% and Re: from 0.02 to 0.5%, and the balance substantially Ni and incidental impurities; and
It is also known that blades of dynamic machines, such as rotor and stator blades of gas turbines, rotor blades of hot-gas blowers and so forth, are made from columnar Ni-base heat-resistant alloy castings. Such a columnar Ni-base heat-resistant alloy casting is produced by a process having the steps of: preparing a melt of an Ni-base alloy by vacuum melting, pouring the melt into a mold of a uni-directional solidifying apparatus, and moving, while the mold is being heated to a temperature of from 1480 to 1530.degree. C., the mold on a chill plate at a moving speed of from 200 to 350 mm/h downward through a water-cooled chilling apparatus so as to allow the columnar crystals formed on the chill plate to grow, whereby a large-size elongated cast article or a large-size elongated turbine blade of columnar Ni-base heat-resistant alloy is obtained.
In recent years, gas turbines are becoming larger in size, which has given a rise to the demand for turbine blades of greater sizes. Large-size turbine blades made of columnar Ni-base heat-resistant castings, cast from conventional Ni-base heat-resistant alloy, however, undesirably exhibit rough cast surfaces, as well as local defects in the form of convexities and concavities in the surfaces. Thus, it has been impossible to produce large-size turbine blades of Ni-base heat resistant alloys having sound cast surfaces. Roughness and local defects appearing on the outer surface of the cast large-size turbine blade do not pose any critical problem, because the surface can be smoothed and the local defects can be removed by grinding and polishing. However, no means are available for smoothing inner surfaces of large-size turbine blades formed by a core mold, nor for removing local defects on these inner surfaces. A high degree of roughness on the turbine blade inner surfaces, as well as local defects, tend to trigger a rupture and to reduce creep fatigue strength, thus impairing the reliability and life of the turbine blade.
Production of turbine blades of greater sizes, made of columnar Ni-base heat-resistant alloy casting, also tends to allow generation of a multiplicity of micro-pores in the internal structure of the columnar Ni-base heat-resistant alloy casting. Thus, it has been impossible to produce large-size turbine blades having an acceptably small number of micro-pores in the structure, from columnar Ni-base heat-resistant alloy castings. Conventionally, hot isostatic press (HIP) processing has been effectively used for reducing micro-porosity. Such HIP processing, however, could not completely remove micro-pores generated in the internal structure of the columnar Ni-base heat resistant alloy castings constituting large-size turbine blades. Micro-pores remaining in the internal structure also serves to trigger a rupture and reduces creep fatigue strength, thus impairing the reliability of the large-size turbine blade.
It has also been recognized that production of columnar Ni-base heat-resistant alloy casting, in particular a large-size turbine blade, from a conventional Ni-base heat-resistant alloy tends to allow coarsening of crystal grains, causing a heavy segregation of the alloy components, with the results that intergranular corrosion rapidly proceeds at the grain boundaries where the segregation is most notable. Thus, reliability and life of large-size turbine blades made of a columnar Ni-base heat-resistant alloy casting are impaired due to a serious reduction in the resistance to intergranular corrosion at high temperature.
The segregation of the alloy components, which occurs in large-size turbine blade made of a columnar Ni-base heat-resistant alloy casting of a known Ni-base heat-resistant alloy, also causes a reduction in the mechanical strength. It is therefore necessary to conduct a solid-solution treatment at a temperature higher than that conventionally adopted, so as to promote dissolution of .gamma.' phase which is a precipitation strengthening phase, followed by an aging treatment which causes the .gamma.' phase to be precipitated and dispersed finely. Solid-solution treatment of a columnar crystalline casting of a conventional Ni-base heat-resistant alloy, when conducted at a temperature higher than that used in the known art, causes a local melting of the casting, so that the mechanical strength is seriously impaired, seriously impairing reliability and life of a large-size turbine blade made from such a columnar crystalline Ni-base heat-resistant alloy casting.